The present invention concerns the ground contact system of vehicles, in particular suspension devices, and more particularly the steering of the wheels.
International application WO 01/72572 (corresponding to U.S. Pat. No. 6,688,620), describes a wheel support device which allows a degree of freedom of the camber of the wheel relative to the suspension elements. This degree of freedom is controlled either actively, for example by a piston-cylinder unit as a function of driving parameters of the vehicle, or passively by the forces exerted on the wheel. Patent applications EP1 247 663 (corresponding to U.S. Pat. No. 6,776,426) and EP1 275 534 (corresponding to U.S. Published Application No. 2003/0011157), describe other mechanical principles of a suspension and wheel support device which permit similar operation.
The “wheel plane” means that plane, related to the wheel, which is perpendicular to the wheel axis and passes through the middle of the tyre. The angular position of the wheel plane relative to the body of the vehicle is defined by two angles, the camber angle and the steering angle. The camber angle of a wheel is the angle which, in a transverse plane perpendicular to the ground, separates the wheel plane from the median plane of the vehicle. This angle is positive when the upper part of the wheel is displaced away from the median plane towards the outside of the vehicle, and in this case one speaks generally of “camber” or “positive camber”. Conversely, when the said angle is negative, one speaks of “counter-camber” or “negative camber”. In what follows, “camber” or “camber angle” will be used interchangeably.
The steering angle of a wheel is that angle which, in a horizontal plane parallel to the ground, separates the wheel plane from the median plane of the vehicle.
The camber plane is the plane in which the camber takes place. It is the vertical plane, transverse relative to the vehicle and passing through the centre of the static contact area. When the steering angle of the wheel is zero, the camber plane contains the axis of the wheel.
In application WO 01/72572, which also sets forth the general context of the invention, it is proposed for passive systems that the instantaneous centre of rotation of the camber movement of the wheel relative to the suspension elements should be located below ground level so-that the transverse forces acting in the contact area generate a torque which tends to tilt the wheel plane in the desired direction (this instantaneous centre of rotation is called the “first instantaneous centre of rotation” in the document WO 01/72572). However, although under that condition the transverse forces generate a torque about the camber axis which tends to tilt the wheel in the desired direction, the efficacy in terms of camber variation is very different depending on the configurations created. However, in practice, the sensitivity of the camber variations as a function of the forces transmitted in the contact area is an important criterion. In effect, it is generally sought to design a suspension system such that the passive camber variation is predictable, stable and satisfactory in terms of maximum inclination. This is particularly important for very high-performance vehicles intended especially for racing. For such vehicles the search for absolute performance involves optimisation of the longitudinal and transverse grip. This optimisation is only possible if the camber angle of the wheel is at all times close to the ideal for the functioning of the tyre. A camber that is ideal in terms of tyre grip is one that makes it possible to optimise the homogeneity of the pressure distribution in the contact area, i.e. which for example allows compensation of the effect, on the pressure distribution in the contact area, of lateral deformations of the tyre when it is working with a slip angle (typically when cornering).
Thus, one objective of the invention is a suspension device with variable camber whose passive function is improved.